VSF 2010 Report - Nabo

diagnostic, and those that are non-diagnostic. Diagnostic residues exhibit clear characteristics
that identify the process that they derive from, such as iron production and iron working.
Non-diagnostic residues present no clear characteristics or features that identify them clearly
with a specific technological process. Non-diagnostic slags can often be related to ferrous
metallurgy generally, but it is unclear whether they originate from bloomery-smelting or
smithing. Often, non-diagnostic slag residues can comprise the majority of an assemblage
whereby they are fragmentary, weathered and corroded pieces of slag displaying intermediate
characteristics that prevent them from being assigned to a particular metallurgical process.
Despite previous use of the terms ‘smithing slag lumps’ and ‘cinder’ in other slag reports,
they will not be employed here as will be justified later. Some examples of the slag types
discussed in this section can be found in Appendix 2, along with the figures included in the
text.
Diagnostic residues
Slags that can be associated with a particular process are considered diagnostic. The
diagnostic slags derived from iron production are tap and/or smelting slag (see Fig.1).
Diagnostic iron working slags are smithing hearth bottoms and hammerscale.
Tap Slag
This slag is generated by the bloomery smelting process and has flowed from the furnace in a
liquated state. This may not always have been an intentional act. It is characterised by the
flow texture on the upper cooling surface that has commonly been referred to as a ‘lava-like’
appearance or ‘ropey’ morphology. The upper cooling surface of tap slag exhibits a free
flowing texture consisting of ‘runnels’ or ‘rivulets’. In cross-section and on fractured edges,
tap slag is dense, often with small spheroidal vesicles (bubbles) near the upper surface formed
by escaping gases. There may be a few large voids. The differential in cooling between the
faster cooling surface and the slower cooling bottom can form a two-banded appearance in
section with a fine crystalline fracture. The base, or ‘under-surface’, of tap slag may be rough
and display impressions, sometimes with adhering residues of matter the slag has run upon.
Smelting Slag
Slag that has been formed by the bloomery smelting process that has not flowed from the
furnace can be described as smelting slag. It often has a medium to low density and may
exhibit great variability in its porosity. Vesicles may range in size, frequency and shape, often
having an irregular character. Compared to the free flowing nature of tap slags, smelting slag
is often more viscous in appearance. Sometimes there a long or large viscous flows displaying
the course and direction of the flow. The most characteristic feature of smelting slag is the
presence of large charcoal impressions, and in cases, embedded charcoal. Some smelting slag
may incorporate other residues and inclusions indicative of the smelting environment and
construction, such as stones, technical ceramics (furnace wall or lining) and organic matter.
Some furnaces may produce a ‘furnace bottom’, a dense cake of smelting slag that may
include pieces of ore and fuel, as well as provide an impression of the base of the furnace
structure.
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